In general, mobile communication systems are well known throughout the world, such as various types of cellular phone and car phone, as well as the simple Japanese personal handyphone system (PHS).
In these mobile communication systems, it is very important to measure/analyze the various properties of a modulation signal transmitted/received between a base station and a mobile station (portable terminal), and thereby confirm that the properties are in an allowable range of a predetermined standard.
The measurement/analysis of the modulation signal is roughly divided into frequency analysis and modulation analysis.
First, for the frequency analysis, an occupied frequency range of the modulation signal, transmission power in each frequency, adjacent channel leak power, spurious property, and the like are analyzed.
Moreover, modulation analysis differs with the type of modulation system, however, the items which are analyzed are the modulation factor, modulation precision, and the like.
FIG. 2 shows a constitution of a conventional modulation signal analysis apparatus in which the frequency analysis and modulation analysis can be performed with respect to the modulation signal in this manner.
That is, as shown in FIG. 2, a modulation signal a inputted via an input terminal 1 is inputted to a multiplier (mixer) 2a of a frequency sweeper 2.
A sweep frequency signal b is applied to the multiplier 2a from a sweep oscillator 2b. 
Moreover, the modulation signal outputted from the frequency sweeper 2 is limited in band by a band pass filter (BPF) 3, and subsequently inputted to a multiplier (mixer) 4a of a frequency converter 4.
A local oscillation signal is applied to the multiplier 4a from a local oscillator 4b. 
Therefore, the frequency converter 4 converts a center frequency fC of the modulation signal outputted from the BPF 3 to an intermediate frequency fI.
Moreover, a modulation signal a1 having the center frequency fC converted to the intermediate frequency fI by the frequency converter 4 is inputted to a resolution bandwidth (RBW) filter 5.
Here, as shown in the frequency characteristics diagram of FIG. 6, the RBW filter 5 is controlled with a bandwidth (resolution bandwidth) RBW determined by a resolution in which a frequency component in the modulation signal as an analysis object is set.
The resolution bandwidth RBW is set centering on the center frequency fC equal to the intermediate frequency fI of the frequency converter 4.
After the frequency component is limited in several bands by the RBW filter 5, the modulation signal a1 is subjected to logarithm conversion by a logarithm (LOG) converter 6, and a level signal d of a decibel (dB) unit with the frequency on a time axis (abscissa) is obtained.
The level signal d outputted from the LOG converter 6 is inputted to a video bandwidth (VBW) filter 7.
The VBW filter 7 removes noise included in the level signal d with the frequency on the time axis (abscissa).
Moreover, the level signal d whose noise is removed by the VBW filter 7 is converted to a digital level signal d1 by an analog/digital (A/D) converter 8, and subsequently inputted to a changeover section 9.
On the other hand, the modulation signal a1 whose center frequency fC is converted to the intermediate frequency fI by the frequency converter 4, that is, the modulation signal a1 before the frequency component is limited in several bands by the RBW filter 5 is converted to a digital modulation signal a2 by an A/D converter 11, and subsequently inputted to the changeover section 9.
The changeover section 9 transmits one signal designated by a controller 10 out of the inputted level signal d1 and modulation signal a2 to a waveform memory 11.
The waveform memory 11 stores/retains the inputted level signal d1 or the modulation signal a2.
Moreover, when the digital level signal d1 is stored in the waveform memory 11, an analysis operation section 12 uses the digital level signal d1 to perform frequency analysis.
Furthermore, when the digital modulation signal a2 is stored in the waveform memory 11, the analysis operation section 12 uses the digital modulation signal a2 to perform modulation analysis.
Therefore, a transmission power property calculator 13a, adjacent channel leak power calculator 13b, spurious property calculator 13c, and the like for performing the frequency analysis are disposed in the analysis operation section 12.
Furthermore, a modulation factor calculator 14a, modulation precision calculator 14b, and the like for performing the modulation analysis are disposed in the analysis operation section 12.
Property calculation results in the respective calculators 13a, 13b, 13c, 14a, 14b of the analysis operation section 12 are displayed in a display 15.
Moreover, an operation input section 16 has a function of inputting the aforementioned various measurement items and measurement conditions to the controller 10 by a measuring person (operator).
Furthermore, the controller 10 switches/controls the changeover section 9 in accordance with the measurement (analysis) items inputted via the operation input section 16, and additionally controls a sweep operation of the frequency sweeper 2.
Additionally, the controller 10 selects and starts the respective operators 13a, 13b, 13c, 14a, 14b of the analysis operation section 12.
Moreover, if necessary, the controller 10 changes a bandwidth RBW of the RBW filter 5.
In the modulation signal analysis apparatus constituted in this manner, the pass bandwidth (resolution bandwidth) RBW shown in FIG. 6 in the RBW filter 5 indicates a frequency resolution in a case in which the modulation signal is subjected to the frequency analysis as shown in FIGS. 3A, 3B.
Here, FIG. 3A shows a waveform of the level signal d before being inputted to the VBW filter 7.
Moreover, FIG. 3B shows the waveform of the level signal d before being output from the VBW filter 7.
The VBW filter 7 removes high-frequency noise included in the level signal d in this manner.
FIG. 4A shows transmission power levels of respective channels (n−1), n, (n+1) obtained by the transmission power property calculator 13a of the analysis operation section 12, and leak powers to adjacent channels of the respective channels (n−1), n, (n+1) obtained by the adjacent channel leak power calculator 13b of the analysis operation section 12.
Moreover, FIG. 4B shows a spurious property obtained by the spurious property calculator 13c of the analysis operation section 12.
FIG. 5 shows the modulation precision obtained by the modulation precision calculator 14b of the analysis operation section 12.
In an example shown in FIG. 5, a π/4 quadrature phase shift keying (QPSK) modulation signal is used as the object of analysis of the modulation signal.
In this case, an amplitude error (AS−A) and phase error α(=θS−θ) of an amplitude A and phase θ of a symbol position P measured in an in-phase component (I)/orthogonal component (Q) coordinate system from an amplitude AS and phase θS of a reference symbol position PS are obtained.
As shown in FIGS. 4A and 4B, in order to subject the inputted modulation signal a to frequency analysis, it is necessary to convert the modulation signal a1 to the level signal d with the frequency on the time axis (abscissa).
On the other hand, as shown in FIG. 5, in order to subject the inputted modulation signal a to modulation analysis, it is necessary to directly analyze the waveform of the modulation signal a1 and calculate base band signal components I, Q included in the modulation signal a1. Therefore, the modulation signal a1 before conversion to the level signal d needs to be used.
Moreover, in order to perform modulation analysis, respective signal levels (amplitudes) in respective frequencies in the bandwidth in the modulation signal a1 are preferably substantially constant.
Therefore, when frequency analysis is performed on the inputted modulation signal a, the measuring person (operator) operates the operation input section 16 and selects the digital level signal d1 by the changeover section 9.
Moreover, when the modulation analysis is performed on the inputted modulation signal a, the measuring person (operator) operates the operation input section 16, selects the digital modulation signal a2 by the changeover section 9, and additionally stops a sweep operation of the frequency sweeper 2.
When simple signal changeover means is disposed in this manner, frequency analysis and modulation analysis can be performed with respect to the inputted modulation signal a with one modulation signal analysis apparatus.
However, there is the following problem yet to be solved even in the modulation signal analysis apparatus shown in FIG. 2.
That is, depending upon the modulation system of the modulation signal a as the analysis object, the modulation signal a subjected to multi-channel multiplexing has a predetermined bandwidth centering on the center frequency fC (=intermediate frequency fI).
Therefore, when modulation analysis is performed on the modulation signal a, and excessive band limitation is performed on the modulation signal a, respective base band signals I, Q cannot be correctly demodulated from the modulation signal. Therefore, the modulation precision cannot be correctly measured as shown in FIG. 5.
In order to prevent such situations occurring, in the conventional art, after the modulation signal a1 with the center frequency fC outputted from the frequency converter 4 and fixed to the intermediate frequency fI is subjected to the band limitation with a fixed bandwidth, modulation analysis is performed.
However, for example, in mobile communication systems such as cellular phones, various modulation systems are developed for the modulation system of the modulation signal transmitted/received between the base station and each mobile station (portable terminal), and some of the systems are implemented.
The bandwidth (BW) used largely differs with the respective modulation systems.
For example, as shown in FIG. 6, the bandwidth (BW) used in general personal digital cellular (PDC) phones in Japan is 30 kHz.
Moreover, the bandwidth in the aforementioned PHS and global system for mobile communication (GMS) in Europe is 300 kHz.
Furthermore, the bandwidth in a code division multiple access (CDMA) using a spectrum diffusion system to rapidly increase the number of channels included in one modulation signal is 1.5 MHz, and the bandwidths in W-CDMA are 4 MHz, 8 MHz, 16 MHz, . . . .
In this manner, the bandwidth (BW) for use in the CDMA and W-CDMA using the spectrum diffusion system to rapidly increase the number of channels included in one modulation signal rapidly increases.
Therefore, the bandwidth for performing pass band control on the modulation signal cannot univocally be determined.
Moreover, when a filter for exclusive use in performing the modulation analysis is disposed, the manufacture cost of the modulation signal analysis apparatus is much increased.